The present invention relates to an AlGaAs surface polishing method for polishing the surfaces of AlGaAs layers formed on GaAs substrates by epitaxial growth or AlGaAs substrates to afford a planar mirror-surface thereto.
In general and in most cases, the AlGaAs substrates used as high-luminance LED chips have industrially been obtained by forming AlGaAs layers on GaAs substrates by epitaxial growth, followed by the removal of the GaAs substrates of light absorptivity. In this case, since the GaAs substrates have a lattice constant of 5.6533 .ANG. whereas, for instance, Al.sub.0.7 Ga.sub.0.3 As has a lattice constant of 5.6588 .ANG., a number of misfit transitions occur on the surface thereof due to a difference in the lattice constants.
Some transitions are released since they give rise to cracking in the crystals in the vicinity of the interface during epitaxial growth and take on a mosaic structure; however, another transitions propagate through the epitaxial layer in the the growth direction and reach the surface thereof.
As a result, warping, which has occurred prior to the removal of the GaAs substrates due to a difference in the coefficient of thermal expansion, is eliminated from the AlGaAs substrates after the removal of the GaAs substrates. However, purterbations of the lattice planes still remain in the vicinity of the crystal surfaces, and since the rate of epitaxial growth has a distribution within the wafer plane, irregularities occur on the surface thereof. It is to be understood that in the LPE method, such irregularities are about 10 .mu.m in magnitude.
Not until now has the polishing of AlGaAs been carried out. In order to afford mirror or planar surfaces to AlGaAs, the grown epitaxial layer of AlGaAs has merely been etched to about several .mu.m with Cl.sub.2, etc. in gas systems and with phosphates, sulfates, etc. in wet systems.
Only with etching, however, it has been difficult to produce a uniform and planar mirror-surface in the plane of a large-area wafer having thereon irregularities of as large as ten plus several .mu.m.
For that reason, it is conceived to produce a planar mirror-surface by polishing. However, special attention should be paid to such polishing, since the surface conditions of such AlGaAs substrates are not substantially found in ordinary GaAs wafers.
The polishing liquid used for polishing serves to uniformly oxidize AlGaAs, but should not allow the oxidation reaction to proceed further after the formation of an oxide film.
Since the surfaces of the AlGaAs substrates are not always formed of the same homogeneous lattice planes due to their perturbations, irregularities may become more noticeable by reason that there is a difference in the reaction with the polishing liquid due to a difference in the crystal lattice planes.
In a case where the reaction continues to proceed to such an extent that the formed oxide film dissolves in the polishing liquid, irregularities again become more noticeable, thus making it impossible to obtain a planar mirror-surface.
An alternative polishing method widely available for Si wafers, GaAs wafers and the like makes use of several types of abrasives differing in particle sizes. However, this method is impractical for the AlGaAs substrates on the order of 200 .mu.m, since there is a need of leaving a margin of several .mu.m or more for the purpose of preventing the abrasives for causing damage thereto.